Hydraulic transmissions utilizing relative open and closed conditions of a pair of valves to establish a particular gear ratio within a range of gear ratios available from a hydraulic transmission for applying different torques for example for rotatably driving the drive shaft, of a motor vehicle are well known.
The valves are generally ported and feature displacements such that when both are closed or open or one is closed and the other is open or vice versa a particular gear ratio is established for each condition by means of hydraulic actuation and deactuation of various clutches contained within the transmission.
Although the relative position of such valves have in the past been controlled mechanically by various shift mechanisms or hydraulically to provide a completely automatic transmission such as disclosed for example in U.S. Pat. No. 4,467,675, a need has existed to provide an electrical interface between the valves and the mechanical components of the shifter to provide the operator with improved feel for the various gear positions yet enable the operator to shift from one gear position to another in a smooth and responsive manner by means of a mechanical component and electrical interface that is simple in its design and economical to manufacture.
In addition to the hereinbefore described need for an economical electro-mechanical shifter for a hydraulic tranmission of the two controlling valve type, there exists a need to provide electro-mechanical shifting for a hydraulic transmission having a clutching requirement for changing back and forth between separate groups of gear ratio ranges as well as provide interlocking clutch restrictions on movement of the shifter between such groups of gear ratio ranges to provide protection to the gear train involved in the transmission as well as the output shaft for example of a motor vehicle being driven by the transmission from undue shock and torque load changes.